One previous method for making oxides of bismuth included comminuting the bismuth metal into fine particles. The fine particles are subjected to a burning flame. Combustion of the bismuth particles results in bismuth oxides. The particles are collected using filters. A disadvantage to this process is the incomplete combustion of some of the larger particles leading to the formation of slag.
Another process for making oxides of bismuth included using a sealed vessel divided by a partition wall into two compartments. One of the compartments contains a furnace for heating bismuth. An inert gas is fed into this compartment to attain a specific bismuth vapor concentration. The inert gas is said to avoid the formation of oxide on the surface of the molten metal which prevents vaporization of the bismuth. The vapors then pass into the second compartment where air or oxygen enriched air is provided to form the bismuth oxides. The bismuth oxides are discharged from the sealed vessel and collected.
Another process for making oxides of bismuth includes adding a monocarboxylic acid to an aqueous solution containing trivalent bismuth ions thereby inducing formation of a bismuth monocarboxylic acid complex in the solution. Adding an alkali to the solution induces precipitation of the bismuth monocarboxylic acid complex. The precipitated complex is separated and fired at a temperature of 340° C. to 360° C. for about two to four hours to produce the bismuth oxides.
In another process of making oxides of bismuth, a nozzle is employed to direct a blast of gas at the molten metal as it is vaporized using an electric arc. Directing the gas at the surface of the molten metal causes dross to form thereon. Formation of dross is undesirable because it reduces the surface area from which vaporization of the metal can occur. In order to increase the area from which the vaporization can occur, it is suggested that the dross be scraped from the surface of the molten metal. In addition, this prior process describes producing bismuth oxide from small amounts of bismuth on the order of 3 to 4 grams.
In general, other than the wet method and the method employing an electric arc, the prior art methods of making bismuth oxide rely on low temperature conventional furnaces to heat the bismuth to the point of melting. These furnaces use heat generated by electrical resistance or combustible fuels. The earlier method lead to the formation of slag, dross, or contaminants on the surface of the molten bismuth necessitating the need for inert gases and a two-chambered vessel of the later process. The inert gases and two-chambered vessel add to the cost and complexity of making bismuth oxides. The method employing an electric arc on small quantities of metals, i.e., 3 to 4 grams, also suffers from the problems associated with dross formation.
Accordingly, there is a need for a newer method of converting bismuth to bismuth oxide on a larger scale, e.g., greater than 100 pounds of bismuth, without having any of the aforementioned drawbacks. The present invention fulfills this need and has further related advantages.